This invention relates to an a pawl for an actuator mechanism for a drum brake wherein an adjustment for a brake application is limited to an increment defined by a pitch of a plurality of teeth on a star wheel.
U.S. Pat. Nos. 4,220,227 and 4,502,574 disclose adjuster mechanism for use in a drum brake to control the extension of a strut member and define a running clearance between a friction surface on brake shoes and a drum member. In particular, the adjuster mechanism includes a pawl carried by a first brake shoe wherein a spring is connected to the pawl and the other brake shoe such that movement of the first brake shoe toward the drum causes the pawl to pivot on the first brake shoe. When a predetermined movement of the brake shoe occur as when lining wear occurs, the pawl pivots to an extent that a first edge moves past a tooth on a star wheel and when the brake application terminates, the first edge again engages the tooth and rotates the star wheel to cause an extension in the strut member. This type adjuster mechanism functions in an adequate manner for most operations, however, should the drum brake be operated under extreme conditions wherein an input force is applied to moves the friction surface on the brake shoe into engagement with the drum it is possible that the first edge on the pawl moves past several teeth on the star wheel. Now on return of the pawl to a rest position, a second edge engages the star wheel and rotates the star wheel through an arc corresponding to several teeth such that a desired running clearance is largely reduced or non-existent. If the running clearance is totally eliminated, the brake will drag until the friction lining is worn away through continued engagement with the drum. Thus, it is important to control the rotation of the star wheel so that the extendable strut is not over adjusted and the brake shoes are maintained with a desired running clearance with respect to the drum.
A primary object of the present invention is to provide a pawl for an adjuster mechanism that limits an adjustment of an extendable strut member to a single increment defined by a pitch of a plurality of teeth on a star wheel no matter how far an input force moves a brake shoe during a brake application.
According to this invention, the adjuster mechanism for a drum brake of a vehicle including an extendable strut that is located between a first web of a first brake shoe and a second web of a second shoe to define a running clearance between said first and second shoes and a drum. The first and second brake shoes are retained on a backing plate that is secured to the vehicle such that a first engagement end of the first web and the second web is aligned with an anchor on the backing plate and a second engagement end is spaced apart by an actuator member. A first spring attached to the first and second webs urges the first engagement ends toward the anchor while a second spring is attached to a lever of the adjuster mechanism that is retained on the first web and said second web. The second spring urges the second engagement ends toward the actuator member such that the first and second brake shoes are positioned with respect to the drum to define the running clearance when the actuator is in a rest position. The actuator responding to an input force by acting on the first and second webs in opposition to the first and second springs and moves the first and second brake shoes through the running clearance and into engagement with the drum to effect a brake application. The adjuster mechanism including a shaft with a star wheel located between a first threaded section and a smooth section with the smooth section being retained in a bore of a first body of the extendable strut while the first threaded section is mated a second threaded section in a second body of the extendable strut. The lever pivots on the first web with movement of the first web with respect to the actuator and rotates the star wheel when a predetermined pivotal movement occurs that causes a corresponding rotation of the first threaded section with respect to the second threaded section such that the extendable strut expands to maintain a desired minimum running clearance. The star wheel is characterized by a plurality of teeth while the lever is characterized by a base with a first slot for receiving a pivot pin secured to the first web for securing the base to the first web, an opening for receiving a first end of the second spring, a second slot for receiving a first end of the first body of the extendable strut and an arm extending from the base to align a first blade thereon in perpendicular alignment with a first tooth of the plurality of teeth on the star wheel and to align a second blade thereon in a plane that is substantially parallel with a pitch of a second tooth of the plurality of teeth on the star wheel. The first blade engages the first tooth with lever and first web are positioned in the rest position while the second blade is offset a distance from the second tooth. The base pivots on the pivot pin during a brake application and when the first web moves a predetermined distance toward the drum the lever will have pivoted through a predetermined angle such that first blade slides past the first tooth and moves into engagement with a third tooth adjacent said first tooth while said second blade moves through the offset distance and into engagement with said second tooth such that said lever is now locked into engagement with the star wheel. The first blade is thereafter prevented from moving past the third tooth and the lever and star wheel move together should the lever continue to pivot with further movement of the web toward the drum. When the input force terminates, the second spring return the lever to the position of rest and the first blade now engages the third tooth to rotate the star wheel an increment defined by the pitch of the plurality of teeth as the second blade moves away from the star wheel the offset distance. Thus, the rotation of the star wheel is limited to a single increment to prevent over adjustment of an extendable strut during a brake application.
An advantage of this invention resides in a means whereby a pawl is locked to a star wheel when a predetermine movement of a web member occurs such that any additional movement does not effect later the rotation of the star wheel to extend an extendable strut and set a running clearance between a friction surface and a drum.